1. Field of the Invention
This invention relates to an open-loop and/or closed-loop control device for a 3-phase power converter for operating an asynchronous machine. The invention further relates to a corresponding open-loop and/or closed-loop control method. In particular, the invention relates to the controlled and/or regulated impression of a torque reference value and a stator flux reference value for a converter-operated three-phase asynchronous machine. The invention is intended in particular for use in heavy-duty applications such as, for example, traction power converters for the supply of power to propulsion motors of railway vehicles.
2. Description of the Related Art
One characteristic of such propulsion systems is the use of three-phase asynchronous machines in connection with three-phase pulse-controlled converters and impressed intermediate current voltage. These propulsion systems, on account of the required high power density and required efficiency, are operated at a relatively low operating frequency. For example, the operating frequency in the voltage operating range is only in the range of 300 Hz to 800 Hz on locomotives for main-line trains, railcars, motor sets and heavy-rail commuter trains. In light-rail commuter trains, the operating frequency typically lies in the range of 800 Hz to 3 kHz. The available intermediate circuit voltage must be utilized optimally, i.e. the open-loop and/or closed-loop control structure must make possible operation in the field weakening range without any operating voltage reserve. To avoid unacceptable system perturbations, it is also necessary to generate a defined and controllable steady-state harmonic spectrum. Together with the relatively low operating frequency and maximum output voltage of the power converter, this requires the use of synchronous pulse control methods.
The requirements for the control response of traction units powered by current converters are also relatively demanding. Normally, on a propulsion system in the lower and intermediate power range, power converters with a relatively high operating frequency (5 kHz to 20 kHz) are used in connection with a conventional field-oriented control method to meet control requirements of this type.
For traction applications, in particular for direct operation on a direct-current trolley line without an input controller, it is important to have a good disturbance reaction of the closed-loop control system to sudden changes in the trolley line voltage. The control of skidding and slipping actions as well as the attenuation of mechanical propulsion vibrations and stable operation require an effective and highly dynamic control response of the indirect torque impression compared to steady-state drives of the same power class.
The closed-loop and/or open-loop control method for the protection of the power converter and/or of the motor must also guarantee a predictable maximum current load and securely prevent the commutation failure of the connected three-phase asynchronous machine as well as individual machines within a group drive. This requirement also applies in particular when there are variations in the disturbance and reference variables for the reasons described above.
The prior art describes methods that are used in particular under the boundary conditions indicated above. One feature common to the methods of the prior art is the division of the individual open-loop and/or closed-loop control method into the essential functions: measured data acquisition, flux model, control structure and trigger equipment (for the pulse generation), whereby a distinction is made in particular between the closed-loop control structure and the open-loop control equipment for the individual methods listed above. Some or all of the above mentioned functions are conventionally realized inside a signal processor system and to some extent with direct FPGA (Field Programmable Gate Array) support.
In general, the following analog measured variables are measured for the closed-loop drive control systems described above:                At least two of the three power converter phase currents (machine current or sum of the individual machine currents in group drives with machines connected in parallel) and        The intermediate circuit voltage of the pulse-controlled converter.        In one possible variant, two phase voltages of the power converter output can also be measured.        Optionally, the individual motor temperatures can also be measured and used for, among other things, the tracking of the resistance parameters of the machines as a function of the temperature.        The motor speed can also be measured as an additional measured variable.        If one power converter feeds two or more traction motors which are connected in parallel, the individual motor speeds are preferably acquired individually and the arithmetic average, for example, can be used for the closed-loop control.        
The basis for the field-oriented closed-loop control methods of the prior art is the knowledge of the magnitude and the angular position of the rotor flux in rotor-flux oriented methods and/or of the stator flux in stator-flux based methods. But because the flux linkages and the torque of the machine cannot be measured directly, mathematical models (flux models) which simulate the internal structure of the asynchronous machine are generally used.
A flux model can be used in particular for the determination of the flux from the measured values or from values simulated by means of suitable calculation processes for the machine terminal voltage, machine phase current and speed. Conventionally, the flux model is composed of two known sub-models of the asynchronous machine, namely the voltage model and the current model. At lower speeds, the influence of the current model predominates, while at higher speeds, on the other hand, the influence of the voltage model is predominant. The combination of the two sub-models combines their advantages and can be interpreted as a voltage model guided by the current model.
The method described in Stanke, G., Horstmann, D.: “Die stromrichternahe Antriebsregelung des Steuergerätes für Bahnautomatisierungssysteme SIBAS32” [Closed-loop drive control via the power converter of the control unit for SIBAS32 railway automation systems], eb-Elektrische Bahnen, Volume 90 (1992), No. 11, pp. 344-350, is a rotor-flux-oriented control method based on mean values with asynchronous and synchronous pulses for the actuation of the power converter. Along with the advantages of, among other things, the synchronous pulses and of the indirect two-component current control system (see above), this method has disadvantages in terms of the achievable control response and disturbance response, in particular in response to variations in the intermediate circuit voltage.
The DSR method described in Depenbrock, M.: “Direkte Selbstregelung (DSR) für hochdynamische Drehfeldantriebe mit Stromrichterspeisung” [Direct automatic control (DSR) for highly dynamic rotating field drives with power converter feed], etzArchiv, Vol. 7 (1985) No. 7, pp. 211-218 and in Jänecke, M., Kremer, R., Steuerwald, G.: “Direkte Selbstregelung, ein neuartiges Regelverfahren für Traktionsantriebe im Ersteinsatz bei dieselelektrischen Lokomotiven” [Direct automatic control, an innovative control method for traction drives used for the first time in Diesel-electric locomotives], eb-Elektrische Bahnen, Vol. 89 (1991), No. 3, pp. 79-87 is a method based on instantaneous values which is particularly well suited for traction drives and has an optimal dynamic response, among other things, although it does not have a reproducible steady-state response. The DSR direct automatic control system also allows only very small ratios of the operating frequency to the fundamental frequency. Among other things, moreover, on account of the minimum operating period of the power converter, operation at low speeds is problematic, a problem that can be solved by switching to an alternative, likewise stator-flux based control method called ISR (indirect automatic control) (see the above referenced publication by Jänecke, M. et al.).
The DTC method which is described in “Direkte Drehmomentregelung von Drehstromantrieben” [Direct closed-loop torque control of three-phase drives], ABB Technik, No. 3, (1995), pp. 19-24, is a method based on instantaneous values which offers an optimal dynamic response, like the DSR automatic control method. However, the steady-state response is likewise not reproducible, and this direct torque control system also does not permit very small ratios of operating frequency to fundamental frequency. In contrast to the DSR, in the DTC method, the stator flux trajectory follows a circular path which, among other things, requires a significantly higher operating frequency of the power converter.
In the methods described in WO 2005/018086 A1, in Amler, G.; Hoffmann, F.; Stanke, G.; Sperr, F.; Weidauer, M.: “Highly dynamic and speed sensorless control of traction drives”, Proc. EPE Conference 2003, Toulouse, in Evers, C.; Hoffmann, F.; Steimel, A.; Wörner, K.: “Flux-guided control strategy for pulse pattern changes without transients of torque and current for high power IGBT-converter drives”, Proc. EPE Conference 2001, Graz and in Wörner, K.: “Quasi-synchrone statorflussgeführte Pulsverfahren für die wechselrichtergespeiste Induktionsmaschine” [Quasi-synchronous stator-flux guided pulse control methods for the induction machine operated by a power converter], Dissertation 2001, VDI-Fortschrittsberichte, Series 21, No. 302, the disadvantages described above such as a poor control response and poor disturbance response at relatively low operating frequencies of the mean-value based control processes with a downstream pulse pattern generator are eliminated by a stator-flux guided pulse generation based on instantaneous values.
D1 (G. Griva et al.) describes a field weakening method for induction motors, whereby a DTC (Direct Torque Control) system is used. In the DTC system, the stator flux and the torque are controlled. Switching pulses for the power converter switch are received via the space vector PWM method. According to D1, the torque and the stator flux are dead-beat controlled.
D2 (Tripathi et al.) describes the dynamic and stationary response of a torque control system in the field weakening range for a DFC (Direct Flux Control) method which uses a stator-flux vector based space vector modulation. In this method, a reference value for the stator frequency is prepared in an external loop as the result of a combination of the output variable of a torque controller and a measured value for an internal loop. The internal loop has a predictive stator flux control with a dead-beat response and a space vector modulation based on a stator flux error vector.
D3 (Lee et al.) describes the introduction of a dead-beat control method for a conventional direct torque control system. Prior art publication D3 does not describe a closed-loop control structure.